Areal density improvements have been a main driving force in the progress of magnetic recording technology. Typically, disks (media) in each new disk drive product have a higher signal to noise ratio and the ability to record sharper magnetic transitions than that of previous products. Even a small improvement in signal to noise ratio may significantly impact the recording performance and therefore the areal density of the media.
Currently, magnetic media evaluation is generally executed by spin-stand tester systems during the media development cycle. The parametric data tested includes overwrite (OW), half-peak pulse width (PW50), track average amplitude (TAA), DC erased signal to noise ratio (DCSNR), spectrum signal to noise ratio (SpSNR), 4TSNR, 2TSNR, 1TSNR, where 1T stands for bit period of written data at high frequency and 4T means ¼ high frequency and SNR stands for signal to noise ratio. This data is generally measured for each disk and then ranked for media design optimization.
However, it is often the case that media ranked based on these parametric data is not well correlated with hard disk drive (HDD) file data. The most critical parameter in HDD file data is byte error rate (BER). As shown in FIGS. 1A and 1B, the BER of a series disk does not correlate well with media component test parameters, in this case SpSNR. Thus, a bit error rate test is needed to more fully evaluate the media. There are many varieties of BER tests that have been proposed.
U.S. Pat. No. 6,157,507 describes a performance evaluation method of linking PW50 and SNR to the BER via equation (2) in column 1 and equation (1) in column 4 of the patent. The relationship between BER and SNR is illustrated in FIG. 9 of the patent. For the disk media used in high density (>100 Gb/in2) and high data rate (>500 Mb/s) applications, the performance evaluation method and the described simplified relationship between BER and SNR in this prior art are not as accurate for testing these more modern disk drives. The present invention advantageously uses a more accurate alternate to SNR to determine the BER.
U.S. Pat. Nos. 5,490,091 and 5,355,261 constructs an algorithm for a partial response maximum likelihood (PRML) data detection channel. This algorithm allows BER to be directly measured. This patent describes PRML chip design and the method for making a PRML integrated circuit (IC) chip for applications in magnetic data storage systems. A PRML chip is a hardware component in a hard disk drive. The present invention is advantageously designed to test a disk drive faster than a device using a PRML chip.
U.S. Pat. No. 5,121,263 illustrates an algorithm for a PRML data detection channel to directly measure BER. The patent describes a BER evaluation for component-level disk media testing without using an extra hardware.
The BER tests of the current art have many drawbacks. Current BER tests are usually time consuming since they usually use more than 5 head gimble assemblies (HGA) to test the same disk media surface. The average result of these HGA is defined as the BER for this specific surface. An example is shown in the FIG. 1A, where 5 heads are used, and their average BER value is referred as BER for the disk surface e.g. BER=−7.61 for surface A of disk ID 9312k. The tests are time consuming, taking as much as seven minutes, for a variety of reasons. First, the tests include writing millions of transitions with a variety of bit patterns, and then reading them back while counting the number of error occurrences. Second, it requires a read channel chip optimization to perform the BER tests. The data channel optimization involves complex procedures such as read/writer interface, read interface timing, sector format, servo interface and circuit description, register bit description, detector polynomial control, AE write gate control, servo sequence control, dibit transfer count, read gate timing loop control, and equalization etc.
Other limitations of current BER tests also exist. For instance, a further limitation of the BER test is that accuracy of BER data is strongly dependent on head stability. This dependency may provide false disk media BER results due to head degradation. In addition head crashes and degradation may adversely affect BER test results. Lastly, BER data is often not correlated well with SNR measurement. Therefore, a new testing method for media performance that is accurate and cost effective is needed.